The present invention relates to a coloured soda-lime glass comprising the main glass-forming constituents and colouring agents.
The term “soda-lime glass” is used in the broad sense here and relates to any glass containing the following constituents, the quantity of which is expressed as a percentage relative to the total weight of glass:
Na2O10 to 20%CaO 0 to 16%SiO260 to 75%K2O 0 to 10%MgO 0 to 10%Al2O30 to 5%BaO0 to 2%BaO + CaO + MgO10 to 20%K2O + Na2O 10 to 20%.
This type of glass is very widely used, for example, in the glazing for the building or automotive sector. It is currently manufactured in ribbon form using the float process. Such a ribbon can be cut into sheets which can then be bended or subjected to a treatment to reinforce their mechanical properties, e.g. thermal toughening.
It is generally necessary to relate the optical properties of a glass sheet to a standard illuminant. In the present description 2 standard illuminants are used: illuminant C and illuminant A as defined by the Commission Internationale de I'Éclairage (CIE). Illuminant C represents the average daylight having a colour temperature of 6700 K. This illuminant is especially suitable for evaluating the optical properties of glazing intended for building as well as the colour of glazing for motor vehicles. Illuminant A represents the radiation of a Planck radiator at a temperature of about 2856 K. This illuminant represents the light emitted by vehicle headlights and is essentially intended for evaluation of the optical properties of glazing intended for motor vehicles. The Commission Internationale de I'Éclairage has also published a document entitled “Colorimétrie, Recommandations Officielles de la C.I.E.” [Colorimetry, Official Recommendations of the CIE], (May 1970) which describes a theory according to which colorimetric coordinates for the light of every wavelength of the visible spectrum are defined so that they can be represented on a diagram with orthogonal axes x and y, referred to as the CIE 1931 trichromatic diagram. This trichromatic diagram shows the representative location of the light of every wavelength (expressed in nanometres) of the visible spectrum. This location is referred to as the “spectrum locus” and the light with coordinates located on this spectrum locus is said to have 100% excitation purity with the appropriate wavelength. The spectrum locus is completed by a light called the purple boundary which joins the ends of the spectrum locus with coordinates corresponding to wavelengths 380 nm (violet) and 780 nm (red). The area contained between the spectrum locus and the purple boundary is that available for the trichromatic coordinates of all visible light. The coordinates of light emitted by illuminant C, for example, correspond to x=0.3101 and y=0.3162. This point C is considered to represent white light and therefore has an excitation purity equal to zero for any wavelength. Lines can be drawn from point C towards the spectrum locus and any required wavelength, and any point located on these lines can be defined not only by its coordinates x and y, but also as a function of the wavelength corresponding to the line on which it is located and its distance from point C relative to the total length of the wavelength line. Consequently, the hue of the light transmitted by a coloured glass sheet can be described by its dominant wavelength and its excitation purity expressed as a percentage.
The CIE coordinates of light transmitted by a coloured glass sheet will depend not only on the composition of the glass but also on its thickness. In the present description as well as in the claims, all the excitation purity P and dominant wavelength λD values of the transmitted light are calculated on the basis of the specific internal spectral transmission factors (TSIλ) of a 5 mm thick glass sheet with illuminant C at a solid angle of observation of 2°. The specific internal spectral transmission factor of a glass sheet is ruled solely by the absorption of the glass and can be expressed by Beer-Lambert's law:
TSIλ=eEAλ where Aλ is the absorption coefficient of the glass (in cm−1) at the considered wavelength, and E the thickness of the glass (in cm). In a first approximation, TSIλ can also be represented by the formula(I3+R2)/(I1−R1)                where I1, is the intensity of the incident visible light on a first face of the glass sheet, R1 is the intensity of the visible light reflected by this face, I3 is the intensity of the visible light transmitted from the second face of the glass sheet, and R2 is the intensity of the visible light reflected towards the interior of the sheet by this second face.        
The colour rendition index (R), expressed by a number in the range of between 1 and 100, denotes the difference between a colour and the perception that an observer has of it when he/she looks through a coloured transparent screen. The more significant this difference is, the lower the rendition index of the colour in question will become. With a constant wavelength λD, when the purity of the colour of the glass increases, the colour rendition index perceived through this glass decreases. The colour rendition index is calculated according to the standard EN 410. We will refer below to the rendition index of the colour yellow (Rj) of a glass which denotes the distortions of this colour in relation to the colour perceived by an observer looking through this glass.
The following are also used in the following description as well as the claims:                the total light transmission with illuminant A (TLA) measured for a thickness of 4 mm (TLA4) at a solid angle of observation of 2°. This total transmission is the result of integration between the wavelengths of 380 and 780 nm of the term: Σ Tλ.Eλ.Sλ/Σ Eλ.Sλ, in which Tλ is the transmission at wavelength λ, Eλ is the spectral distribution of illuminant A and Sλ is the sensitivity of the normal human eye as a function of wavelength λ;        the total energy transmission (TE), measured with a thickness of 4 mm (TE4). This total transmission is the result of integration between the wavelengths of 300 and 2500 nm of the term: Σ Tλ.Eλ/Σ Eλ. The energy distribution Eλ is the spectral energy distribution of the sun at 30° above the horizon, with an air mass equal to 2 and an inclination of the glazing relative to the horizontal of 60°. This distribution, referred to as “Moon distribution”, is defined in the standard ISO 9050.        selectivity (SE), measured by the ratio of total light transmission for illuminant A to total energy transmission (TLA/TE);        total ultraviolet transmission, measured for a thickness of 4 mm (TUV4). This total transmission is the result of the integration between 280 and 380 nm of the term Σ Tλ.Uλ/Σ Uλ, in which Uλ is the spectral distribution of the ultraviolet radiation which has crossed the atmosphere determined in the standard DIN 67507;        the Fe2+/total Fe ratio, sometimes referred to as the redox ratio, which represents the value of the ratio in atomic weight of Fe2+ relative to the total weight of iron atoms present in the glass, and which is obtained by the formula:Fe2+/Fetotal=[24.4495×log (92/τ1050)]/t−Fe203         where τ1050 represents the specific internal transmission factor of the glass of 5 mm at the wavelength 1050 nm. t−Fe203 represents the total content of iron expressed in the form of oxide Fe2O3 and measured via X-ray fluorescence.        